Make the animationend event listener do not assume that it's always…

… fired for `<dir

According to MDN, `animationend` event is never fired if the event target (`<dir>` in this case)
is removed.  If it's slow environment, `<dir>` element may be removed before the listener receives
the event for it.  Therefore, it should not assume that it always comes.

Differential Revision: https://phabricator.services.mozilla.com/D181961

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1839265
gecko-commit: 8cc85121d9b7c49b9def06d8556709ce75354933
gecko-reviewers: m_kato

editing/crashtests/remove-editing-host-during-forwarddelete.html

@@ -13,12 +13,22 @@
 </style>
 <script>
 document.addEventListener("DOMContentLoaded", async () => {
+  const dir = document.querySelector("dir");
   const waitForAnimationEnd = new Promise(resolve => {
     let count = 0;
     function onAnimationEnd() {
       window.find("AAAAAAAAAA");
       document.execCommand("forwardDelete");
-      if (++count == 2) {
+      count++;
+      function getRemainingEventCount() {
+        if (count >= 2) {
+          return 0;
+        }
+        // If `animationend` is delayed and `<dir>` has already been removed,
+        // `animationend` for it is never fired anymore.
+        return !dir.isConnected ? 0 : 1;
+      }
+      if (!getRemainingEventCount()) {
         window.removeEventListener("animationend", onAnimationEnd);
         resolve();
       }
@@ -27,7 +37,6 @@
   });
   window.find("A");
   document.execCommand("insertHTML", false, "AAAAAAAAAAAAAAAA");
-  const dir = document.querySelector("dir");
   dir.addEventListener("DOMNodeRemoved", event => {
     dir.remove();
   });